TITLE OF THE INVENTION
SINGLE SWEEP MEASUREMENT OF MULTIPLE OPTICAL CHARACTERISTICS
BACKGROUND OF THE INVENTION
The present invention relates to the measurement of optical
characteristics of an optical device under test, and more particularly to a method
and apparatus for measuring multiple optical characteristics in a single sweep of a swept wavelength system using Jones Matrix Eigen Analysis.
It is well known in the art that the Jones matrix of an arbitrary two-port
optical device may be measured by using three known input states of
polarization and measuring the resulting output states of polarization. Polarized
light is represented by a two-element complex vector, i.e., the Jones vector, the elements of which specify the magnitude and phase of the x- and y-components
of the electric field at a particular point in space. The Jones matrix for the
optical device relates the input and output Jones vectors to each other. The
Jones matrix representation is found by measuring three output Jones vectors in
response to three known input stimulus states of polarization, or input Jones
vectors. Fiber Optic Test and Measurement, Dennis Derickson, Prentice Hall,
1998, page 225. The mathematical calculations are simplest when the stimuli
are linear polarizations oriented at zero, forty-five and ninety degrees as shown
in Fig. 1 , but any three distinct stimuli may be used.
Using the convention shown in Fig. 1 the Jones matrix of an optical
device under test (DUT) at a particular optical frequency is calculated from the
following equation:
where the different components of the Jones matrix are given by: K1 = [X1/Y1] K2 = [X2/Y2] K3 = [X3/Y3] K4 = [(K3-
K2)/(K1-K3)] and J[X1 ,Y1] is the output Jones vector for the input linear-horizontal state of
polarization, J[X2, Y2] is the output Jones vector for the input linear-vertical
state of polarization, and J[X3, Y3] is the output Jones vector for the input
linear-forty-five degree state of polarization. In the Jones matrix equation the factor C is a constant phase/amplitude multiplier that is undetermined and unnecessary for measuring polarization-dependent loss (PDL) or polarization differential group, delay (DGD). In practice the output Stokes vector is measured and then the Jones vector is calculated, as is well-known to those skilled in the optical arts as shpwn in the Derickson text book cited above.
Also it is well known that the wavelength-dependent Jones matrix may be measured by sweeping over a wavelength range using a fixed input horizontal state of polarization while measuring the output state of polarization at each
wavelength increment; then sweeping over the same wavelength range using a different fixed input vertical state of polarization while measuring the output state of polarization at each wavelength; and sweeping a third time over the same wavelength range using yet another fixed input state of polarization while
measuring the output state of polarization. Then by correctly registering the sweeps from the various output states of polarization with the same wavelengths
for each sweep, the Jones matrix is calculated at each wavelength using the equations above.
Further it is well known that one may measure the three output states of polarization for three different input states of polarization at a fixed wavelength,
and then calculate the Jones matrix at that wavelength. The wavelength may
then be indexed and the process repeated to calculate the Jones matrix as a
function of wavelength. Knowing the Jones matrix as a function of wavelength
is important because it allows the determination of wavelength dependent
optical characteristics such as polarization-dependent loss (PDL) and
polarization dependent group delay (DGD). These are important characteristics
of optical devices, and help to determine the degree to which the optical device
may degrade an optical telecommunications system. Given the Jones matrix the
PDL may be found from:
PDL = 10*Log(λ1/λ2)
where λ1 and λ2 are the eigenvalues of (J*)TJ. The DGD is also found from the
Jones matrix as:
DGD(ω) = |arg(p1/p2)/Δω|
where pi and p2 are the eigenvalues of J(ω + Δω)*J"1(ω).
It is obvious from these descriptions that the testing over wavelength is
slow. The first process requires three different scans over a wavelength range.
If there are N wavelengths in each scan, then the first method requires the
measurement of N*3 output states of polarization. The second method steps
through the wavelengths only once, but this must be a stepping motion with a
pause at each wavelength to measure the three different states of polarization.
Again the number of output states of polarization is N*3.
What is desired is a faster method of measuring multiple optical characteristics of an optical device that requires fewer measurements of output states of polarization, and more specifically a method of scanning over a wavelength range once to determine the wavelength-dependent Jones matrix of the optical device from which the multiple optical characteristics are calculated simultaneously.
BRIEF SUMMARY OF THE INVENTION Accordingly the present invention provides a single sweep measurement of multiple optical characteristics of an optical device using a swept wavelength system that cyclically changes known input states of polarization on consecutive optical frequencies as the optical frequency is incremented within the wavelength range of the swept wavelength system. From the measured output states of polarization a wavelength-dependent Jones matrix for the optical device is calculated, and from the Jones matrix the multiple optical characteristics are determined, which characteristics may include PDL and DGD.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
Fig. 1 is a graphical view illustrating a common convention for measuring
a Jones matrix for an optical device under test as known in the prior art.
Fig. 2 is a graphical view illustrating the interpolation of wavelength dependent Jones matrix parameters for an optical device according to the
present invention. Fig. 3 is a block diagram view of a system for measuring different optical characteristics of an optical device over a single sweep of optical frequencies over a range of wavelengths according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Table 1 below shows the first nine measurements from a plurality of measurements made for an optical device under test in a single sweep of a swept optical test system over a range of wavelengths ω0-ωn. H stands for an input linear-horizontal state of polarization (SOP), V stands for an input linear-
vertical SOP and'F stands for an input linear-45-degree SOP.
J, is the i
th Jones vector measured at the i
th optical frequency within the swept
wavelength range ω0 . . . ωn. The equations for the components of the Jones matrix at each optical frequency using a linear interpolation are: Kik+i = X/Y, + [(ωi+k - ωJ/(ωi+3 - ω,)rpwYM - X YJ
K2k+i = Xi+1/Yi+ι + [(ωi+k - ωi+1)/(ωi+4 - ωi+1)]*[Xi+ i+4 - Xi+1/Yi+i]
K3k+i = xi+2/Yi+2 + [(ωi+k - ωi+2)/(ωi+5 - ωl+2)]*p YM - xi+2/Yi+2]
K4k+i = (k3k+i - K2k+i)/(K1k+i - K3k+i)
The calculation begins with i = 0 and = 0, 1 , 2. For three input states of polarization k=0 is equivalent to the Jones matrix component at the measured Jones vector for the ith optical frequency at the particular input state of polarization and k = 1 and 2 provide the interpolated Jones matrix components for the same particular input state of polarization at the ith+1 and ith+2 optical frequencies. In other words as shown in Fig. 2 K10 provides the Jones matrix component for the 0th optical frequency (ω0) at the linear-horizontal polarization
(H) based on the measured Jones vector J0, and then K1., and K12 at the first
and second optical frequencies (ω1 and ω2) are derived by interpolating between the measured Jones vectors for the 0th and 3rd (ω3) optical frequencies, J3-J0-
Then i indexes by three and the calculation repeats with k = 0, 1 , 2. Once again i indexes by 3 and k = 0, 1 , 2. This process continues until all the measured Jones vectors for the H polarization have been used. The process is
simultaneously performed for the other input states of polarization, V and F, to obtain the values of K2 and K3. The result is a value for K1 , K2 and K3 for each optical frequency, one value of which is based on a measured Jones vector and the other two of which are interpolated from measured Jones vectors. The
wavelength-dependent Jones matrix at the ith optical frequency is:
This equation represents the desired Jones matrix for the device under test at
each optical frequency over the wavelength range from which the PDL and DGD
equations may be used to determine the respective wavelength-dependent
optical characteristics simultaneously.
Thus the determination of the wavelength-dependent Jones matrix is done using a single scan over a range of wavelengths with a total of N measured
output states of polarization, resulting in a three-fold increase in speed over the
prior art.
As indicated above the components of the Jones matrix between like
states of polarization at the input, i.e., every third optical frequency in this
example, may be interpolated over intervening optical frequencies so that the
multiple optical characteristics are calculated for each optical frequency within
the swept wavelength range. There are many alternate methods of interpolation.
One such alternate method is to fit a curve to the real and imaginary components
of the x and y stεtes of polarization in the Jones vectors for the same input states
of polarization listed in Table 1 , and from this fitted curve template the values of
the Jones vectors at intervening optical frequencies may be determined for such
state of polarization. For example the output Jones vectors for horizontally
polarized light H are measured at optical frequencies ω0, ω3, ω6, etc. By plotting
the components of the output Jones vector at these frequencies, and applying a
curvilinear fit, the values of the components of the Jones vectors for horizontally
polarized light at ω1( ω2, ω4, etc. are determined. A similar procedure may be used for each input state of polarization. Through this procedure the output state of polarization at any optical frequency for the three input states of polarization are determined, and from these the Jones matrix may be calculated at each optical frequency, as indicated above. Then DGD and PDL are determined, also as indicated above. The interpolation may be a linear interpolation of ratios as in the equations for K1k+i, K2k+i and K3k+i above, a curvilinear interpolation of these ratios, a curvilinear interpolation of the components of the Jones vectors, etc.
A typical test system is shown in Fig. 3 where a swept optical source 12
provides an optical signal which increments or scans in optical frequency over a designated wavelength range. The optical signal is input to a polarization controller 14 that changes the state of polarization of the optical signal cyclically
for each incremental change in optical frequency among defined states of polarization, such as linear-horizontal, linear-vertical and +linear-45-degree states of polarization as shown in Table 1 above. The resulting polarized optical signal is then input to the optical device under test (DUT) 16. The optical signal
output from the DUT 14 is input to a polarimeter 18. The measured outputs from
the polarimeter 18 are input to a processor 20, such as a digital signal processor
(DSP), where the Jones matrix for each optical frequency of the optical signal
over the wavelength range is calculated as indicated above. The DSP 20 then
provides as outputs the wavelength-dependent optical characteristics of the DUT 16, such as PDL and DGD, determined from the Jones matrices. Although three
specific linear states of polarization are referenced here, any three distinct states
of polarization may be used from which the Jones vectors may be measured.
Thus the present invention provides a method of performing a single sweep simultaneous measurement of multiple optical characteristics of an optics device using a swept wavelength optical system by cyclically changing the input
states of polarization of an optical signal on consecutive optical frequency increments of the wavelength scan and measuring the output state of polarization from the optical device, from which measurements a wavelength dependent Jones matrix is calculated, the wavelength dependent Jones matrix then being used to determine the multiple optical characteristics of the optical device simultaneously.